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*Corresponding author:
Email: mcmtega@yahoo.co.uk, +255-753122642 https://dx.doi.org/10.4314/eajbcs.v4i2.5S

Evaluation of Toxicological Risks and Effects of Microplastics on Nile Tilapia (Oreochromis
niloticus) under in Vitro Laboratory Conditions

Macarius Pancras Mtega1*, Matobola Joel Mihale1 and Kessy Fidel Kilulya2

1Department of Physical and Environmental Sciences. Faculty of Science, Technology and
Environmental Studies. Open University of Tanzania

2Department of Chemistry, College of Natural and Applied Sciences, University of Dar es Salaam

KEYWORDS:

Microplastics;
Toxicological risks;
Oreochromis niloticus;
Aquatic organisms;
Biological effects;
In vitro condition

ABSTRACT

Microplastics have been reported by many literatures as contaminants of environmental
water bodies and are ingested by aquatic organisms due to their small sizes. Knowing the
effects of microplastics to fresh water fish which are kept in ponds helps in managing fish
keeping practice. The objective of research was to determine the toxicity of microplastics
to Nile tilapia (Oreochromis niloticus). The Experiment was done in 80 fish samples.
Microplastics which were prepared for the batch experiment were introduced in the
aquarium followed with observation for 21 days. The digestion of fish gills and intestines
involved 10 mL of 10% (w/v) KOH solution and incubation at 65oC for 24 hours.
Engulfed microplastics were determined using stereo microscope and At-IR
spectrophotometer for confirmation. Engulfed microplastics were observed to be in mean
range of 3.37 x 102±4.04 x 102 to 2.32 x 103 ± 3.57 x 103 particles/kg in gills and 4.68 x
104 ± 3.02 x 104 to4.40 x 104 ± 5.34 x 104 particles/kg in intestines. The observed
responses were loss of equilibrium for 35% of fish, abnormal swimming for 49% of fish,
abnormal ventilator behavior for 59% of fish, abnormal appearance for 39% of fish and
average growth weight increase in control experiment fish was 6.10±2.62 g compared to
1.7 ± 3.62 g in test fish. There was no mortality of Nile tilapia. The responses of fish to the
presence of microplastics in aquarium indicated that microplastics had adverse effects to
Nile tilapia (Oreochromis niloticus). More researches have to be done on fish
physiological changes caused by microplastics.

INTRODUCTION

Plastics are produced in large amount for
different applications, but have been
accompanied with waste deposition in urban
areas (Moore, 2008; Zhang, 2017). About 70

to 80% of plastic contaminants originate from
anthropological activities (Mvowiec, 2017).
Plastic wastes in environments occur in
different sizes, the smallest forms are called
microplastics (Sadri and Thompson, 2014;
Thakur et al., 2022). When large proportions

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of plastic wastes are mismanaged, they enter
the environments where they evolve into
microplastics via progressive fragmentation
through the process of bio-degradation, photo-
degradation, thermo-oxidative degradation or
mechano-chemical degradation (Mvowiec,
2017; Guilhermino, 2021).

Microplastics have been found in marine biota
because of their presence in ecosystems
(Mistri, 2022). Animals exposed to
microplastics under laboratory setting have
shown several adverse effects like histological
alterations, lesions in the gastrointestinal tract,
intestinal inflammation, neurotoxicity,
oxidative stress, damage, immuno-regulation,
feeding behavioral change, and developmental
alterations (Jovanovic, 2017). Oxidative stress
and inflammation are caused by generated
reactive oxygen species (ROS)
(Subaramaniyam et al. 2023;Yao et al., 2023).

In urban areas, there are various
anthropogenic activities which lead to
microplastic accumulation in water bodies in
which fish are found living (Khan et al., 2018;
Mayoma et al., 2020). Fish can be easily
stressed due to exposure to toxic substances in
water (Reverter, 2018). The reported clinical
signs by literatures on fish responses to
abnormal water conditions have not been well
documented on fresh water fish commonly
known as Nile tilapia (Oreochromis niloticus)
once exposed to microplastics. Fish keeping is
highly encouraged and takes place in ponds in
various tropical regions in Africa where
Tanzania is among of them. With knowledge
that fresh water ponds are susceptible to
microplastic pollution in urban regions,
therefore there was need of finding out how
far the microplastic exposure to fresh water

fish, especially Nile tilapia could have effect.
The research was conducted to find out how
fish commonly known as Nile tilapia were
affected by microplastics and to evaluate their
effects by in vitro observation in laboratory.

MATERIALS AND METHODS

Materials

An Aquarium was made of glass (30 cm x 30
cm x 60 cm), with a capacity of 40 L. Two
aerators, two air and water filters, a wire mesh
covering, and an aquarium fish net from the
fish equipment business shop were used to
support the aquarium. A laboratory
thermometer and a multimeter for pH, water
conductivity and Total dissolved solids (TDS)
were obtained from the laboratory. Plastic
buckets, (each 20 L), and a sieve were bought
from the domestic shops. Fish feeds were
bought from the animal feed shops. Stereo
microscope, hand lens, and 100 mL beakers
were supplied by the laboratory. Potassium
hydroxide (Analar compound) and Standard
microplastics: Polyethylene (PE),
polypropylene (PP), polyvinyl chloride
(PVC), nylon 6, 6 and polyethylene
terephthalate (PET) were supplied by Precur
Chem and Equipment Ltd, Tanzania.

Samples of fish for toxicological study

Samples of Nile tilapia for microplastic
toxicological study were collected from
Kibaha Fish Farming Ponds in Coast Region,
Tanzania. The selection of this freshwater fish
was based on the fact that it is commonly kept
in most fish farmers in tropical regions due to
the ability of withstanding extreme conditions
like high temperature, change of pH or



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56

changes of total dissolved solids. The fish
have also ability to survive during transport
and the period of caring and observation.
Small fish aged two months old (length 12
mm−14 mm and/ or weight 30 g–50 g) were
captured and collected using nets. The fish
were captured during the sunset and were kept
in ponds within nets until early morning
before sunrise where they were collected in
two buckets of (20 L) each containing 10 fish.
Fish in one bucket was used for test
experiment while in another bucket was for
the control experiment. All the experiments
involving fish samples were performed in
batches of 20 fish and a total of 4 batch
samples were collected. About 4 L of water
from the pond was added in each fish bucket
for maintaining fish medium.

Preparation and aquarium maintenance

The test and control aquaria were set near the
window where sun rays did not struck
directly. Two aerators for each aquarium were
connected to the filters, and then placed in the
aquarium using aquarium fish net. Clean tap
water was half-filled n each aquarium one day
before fish introduction. The water in the
aquarium was left overnight in order to
dechlorinate. In the next day, aeration was
performed to increase the amount of oxygen
and filtration was done for half an hour to
remove some suspended solids. Thereafter,
room temperature, pH, conductivity and total
dissolved solids (TDS) of the water in the
aquaria and the buckets (from the fish ponds)
were measured. Each fish was weighed to
obtain initial weight (w1) and length (l1) prior
to placing in the aquarium. The fish were
placed in the separate aquaria then were left to
acclimatize for 20 min. Then water from fish

farm ponds in the buckets was mixed with de-
chlorinated aquarium tap water which was
already in the aquarium in order to fill the left
space. The purpose of retaining the pond
water was to keep the association bacteria
(symbiosis) used by fish in their life. Lastly,
each aquarium was covered with wire mesh in
order to ensure fish safety. The fish in both
aquaria were starved for one day, then fed
with ground fish feed pellets 10% (w/w). The
fish were left to acclimatize for nine days
before feeding with microplastics. The fish
life was maintained by continuous aeration of
the aquarium water, and measuring pH, water,
room temperature, conductivity and TDS. The
nitrate levels were controlled by dilution with
25% of clean water after every two days. The
gravel was set as aquarium bed in order to trap
suspended solids and feces. The feces were
removed after every two days by siphoning.

Exposure of Nile tilapia to microplastics

Microplastics with size of 30 µm for PVC,
and 1000 µm for PET, 2000 µm for PE, 2000
µm for PP and 600 µm nylon 6,6 were mixed
together in a 250 mL beaker followed with
mixing with water to form a suspension. The
microplastics were prepared in different
concentration dose for exposure to fish; 2.7 x
107, 9.0 x 108, 1.3 x 108 and 6.7 x 107

particles/m3. The prepared micro plastic
suspension was introduced in the test
aquarium on the tenth day. The aquarium
contained fish of average weight 40 g. Then
the observation was performed for 21 days
according to the Organization for Economic
Co-operation and Development (OECD,
2019). The behaviours (responses) of fish in
the test aquarium were compared with the fish
in the control aquarium. The toxic effects of



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57

micro plastics on fish were observed by noting
and recording the number of individuals
affected using some established clinical signs
(Table 1) as stipulated in the OECD (2019).

Furthermore, the physical characteristics of
the aquarium media (pH and conductivity),
weight of fish, and concentration of
microplastics were observed.

Table 1. Clinical signs used in observation of responses of Nile tilapia to microplastics exposure
Observation Clinical Sign
Loss of Equilibrium Abnormal horizontal orientation

Abnormal vertical orientation
Loss of buoyance control (floating at surface or sinking to the bottom)

Abnormal swimming behavior Hypo activity (Decrease in spontaneous activity)
Hyperactivity (Increase in spontaneous activity)
Corkscrew swimming (Rotation around long axis, erratic movement)
Abnormal surface distribution (abnormal depth selection, close to water
air interface
Abnormal bottom distribution/behaviour abnormal depth selection,
bottom of tank)
Over-reactive to stimulus (flight, or avoidance response to: visual, tactile
touch) or vibration stimulus
Loss of schooling/shoaling behaviour (individual fish show, loss of
aggregation and social interaction)
Dense schooling/shoaling behaviour (increase in clumped association of
fish)

Abnormal ventilator
(Respiratory) function

Hyperventilation (increase in frequency of opercula ventilator
movements with possible open mouth and extended opercula)
Hypoventilation (decreased frequency of opercula ventilator movements).
Coughing (fast reflex expansion of mouth and opercula not at water
surface-assume to clear ventilator channels)
Gulping (mouth movements at water surface resulting in intake of water
and air)
Head shaking (rapid lateral head movements)

Abnormal skin pigmentation Darkened (malefic markings)
Lightened (weak pigmentation)
Mottled (discoloration)

Appearance and Behaviour
Abnormalities

Oedema (abnormal swelling due to accumulation of fluids)
Haemorrhage (sub-mucus bleeding)
Faecal (anal) casts (string of faeces hanging from anus or on tank floor)

Source: OECD (2019).

Analysis of engulfed microplastics

After 21 days, all fish from test and
experiment aquaria were weighed for final
length and weight, and then were dissected to
collect intestinal parts and gills. The gills and
intestines were separately digested using 10%
KOH solution (10% (w/v) following with

incubation at 65ºC for 24 hours. Later,
distilled water was added to dilute the unused
KOH before filtration. The particles on the
filter paper were washed thoroughly with
water during filtration. A stereo microscope
(10 x magnifications) was used to count the
microplastics which was performed first by
placing all microplastics in the petri-dish
which had four partitions prepared by lines of



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pen-ink. Secondly, the petri-dish was placed
on the microscope stage for visualization and
counting of microplastics in all petri-dish
compartments (Masura et al., 2015). Further
confirmation of the microplastics was done
using Attenuated Fourier Transform Infrared
Spectrophotometer instrument (At-FT-IR,
Bruker, Massachusetts, USA).

Data analysis

Excel descriptive statistics in analysis ToolPak
was used to analyse row data for mean, range
and standard deviation of microplastics
engulfed by Nile tilapia. Pearson correlation
was used to determine the relationship
between microplastics which were engulfed
and responses of Nile tilapia. Student t-test (n
= 20)was used to determine the significant
difference of effects of engulfed microplastics
on Nile tilapia and significant difference in
growth among the microplastic fed fish (tested
fish) and fish which were not fed with
microplastics (non-tested fish).

RESULTS

Water temperature, pH, total dissolved
solids (TDS) and electric conductivity

Water temperature, electric conductivity, pH
and total dissolved solids were monitored as
part of water quality assurance for fish growth
in both test and control experiments.
Ammonia content in aquarium which was
caused by defecation and food remains was
controlled by diluting water and siphoning out
after every two days of experiments. Dilution
and siphoning were also means of regulation
of pH, TDS and conductivity. The room
temperature was controlled by air circulation

using fans and opening windows. The room
temperature therefore, ranged from 24.8 to
27ºC and water temperature ranged from
24.00 to 26.00 ºC. The temperature range was
within the water quality requirement for fish
farming, which is 24ºC to 30ºC (Ezeanya et
al., 2015). The water conductivity (mean ±
standard deviation) in tested fish had changed
by 62% (465 ±117.15 µs/cm) from the pond
water conductivity (750±254 µs/cm), the pH
had changed by 0.9% (6.8 ± 0.4) from the
pond pH (6.7 ± 0.2) and TDS had changed by
63.9% (236.5 ±32.4 ppm) from the pond TDS
(370.0 ± 127.3 ppm). The water conductivity
in non-tested fish had changed by 64% (480 ±
80.1 µs/cm) the pond water measurements
were 750±254 µs/cm (electric conductivity),
pH had changed by 0.9% (6.8 ± 0.2) from the
pond pH, 6.7 ± 0.2, and total dissolved solids
had changed by 72% (269.75 ±104.85 ppm)
from the pond TDS, 370.0 ± 127.3 ppm. There
was no significant difference between water
conductivity (t-test, p = 0.7, df = 40), total
dissolved solids (t-test, p = 0.09, df = 40), and
pH (t-test, p = 0.05, df = 40) in test and
control experiments. The recommended pH
range for fish growth is 6−9, electric
conductivity is 100−2000 µs/cm (Ezeanya et
al., 2015) while total dissolved solids is 500
ppm−1500 mg/L (Scannell and Jacobs, 2001).

Microplastic abundance and wet weight
change of Nile tilapia

Microplastics were found in both intestines
and gills of the tested fish in great abundance.
Polyvinyl chloride microplastics were the
most abundant because of the size (30 µm)
which was smaller than other microplastic
sizes. Other microplastics with size 600 µm to
2000 µm were not determined in both gills



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and intestines. The microplastic
concentrations in intestines were in
concentration range of 4.68 x 103 ± 3.02 x
103particles/kg to 4.40 x 104±5.34 x 104

particles/kg (mean ± standard deviation) and
in gills with concentration range of 3.37 x 102

± 4.04 x 102particles/kg to 2.32 x 103 ± 3.57 x
103 particles/kg (Table 3).

The wet weight concentration (mean ±
standard deviation) increase for test fish was
1.7± 0.66–3.66±0.74g and for non-test fish
was 5.2 ± 1.85–6.54 ± 2.11 g. The results
indicated that the non-test fish had great
increase in wet weight compared to test fish
(Table 2) although the statistical comparison
(ONE way-ANOVA) indicated that there was

no significant difference in growth changes
(df = 79, p = 0.9) between test fish  and non-
test fish. The change in growth length was
high in non-test fish. There was significant
difference (p˂0.05, p = 0.00, t-test) in the
effects of microplastics among test fish, also
there was medium correlation (r = 0.3, p =
0.00) between microplastic effects on fish
weight changes. The change in growth weight
of test fish gave scattered cycles which were
underlying straight line to show linear
relationship between the two variables. The
relationship between growth weight change
and microplastics did not depend on
concentration of microplastics, which means
even low concentration could affect negatively
the fish growth (Figure 1).

Figure 1. Relationship between (a) Growth change in weight and microplastic concentration
(b) Growth length change and microplastic concentration

The change in growth length of test fish had a
range of 0.1 ± 0.06 to 1.21 ± 1.16 cm (mean ±
standard deviation) compared to the non-test
which had range of 0.62±0.12 to 1.7±1.0 cm
(Table 2). The mean length growth change in
test fish was smaller compared to non-tested
fish because of some fish which had fin rot
which resulted to loss of tails. The results

indicated that there was small correlation (r=
0.1, p = 0.00) between microplastic effects on
fish length changes. The change in growth
length of tested fish gave scattered cycles
which were underlying straight line to show
linear relationship between the two variables.
The relationship between growth length



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change and microplastics did not depend on concentration of microplastics.

Table 2. Engulfed microplastics in gills and intestines and fed microplastics and Comparison of mean
change in fish weight (g), change in fish length (cm) between tested and non-tested fish in relation to the
engulfed MPs (particles/kg)

1. Engulfed microplastics (mean ± standard deviation) in gills and intestines together with fed microplastics
Exp Exposed Ps/m3 Gills, Particles/kg Intestines
1 2.7 x 107 3.37 x 10 2±4.04x102 4.68 x 103±3.02 x 103

2 9.0 x107 9.95 x 102±9.70 x 102 5.50 x 103±9.31 x 103

3 1.3 x 108 1.79 x 103±1.88 x 103 4.40x 104±5.34 x 104

4 6.7 x 108 2.32x 103±3.57 x 103 1.08 x 104±1.00 x 104

2. Comparison of (mean ±standard deviation) change in fish weight (g), change in fish length (cm) between
tested and non-tested fish in relation to the (mean ±standard deviation) engulfed microplastics
(particles/kg)
Exp Change weight Change in Length Engulfed particles

Tested fish Non-Test fish Tested fish Non-Test fish
1 1.75 ±0.66 5.2 ±1.88 0.46±0.42 0.65±0.34 5.03x103±3.07x103

2 3.66 ±0.74 6.54 ±2.11 0.6±0.18 0.62±0.12 3.58x103±9.94x103

3 2.53 ±2.9 6.52 ±2.32 1.21±1.16 0.8±0.5 4.59x104±5.36x104

4 1.7 ±3.62 6.10 ±2.62 0.1±0.06 1.7±1.0 2.33x104±3.85x104

Mean 2.41±0.92 6.09±0.54 0.57±0.49 0.95±0.53 1.94x104±1.71x104

Biological Effects of Microplastics on Nile
tilapia

The microplastic effect was found in the
fourth day from the test initiation. The fish
which were tested with microplastics and

indicated loss of equilibrium were  4±1 fish
(mean ± standard deviation), abnormal
swimming behavior were 4±2 fish, abnormal
ventilatory function were 6 ± 4 fish and
abnormal external appearance were 3±2 fish
(Table 3).

Table 3. The number (mean ± standard deviation) of Nile tilapia which had positive responses on
microplastic exposure and the observed clinical signs.
Observation Abnormal clinical sign Number of fish

Loss of Equilibrium Horizontal Orientation 4 ± 2
vertical orientation 4 ± 2
Loss of buoyancy control 4 ± 2

Swimming Behavior Hypo activity 4 ± 4
Hyperactivity 5 ± 4
Surface distribution 6 ± 4
Bottom distribution 2 ± 4
Dense schooling 8 ± 1

Ventilatory/Respiration Function Hyperventilation 9 ± 2
Hypoventilation 1 ± 2
Irregular ventilation 6 ± 2
Gulping 9 ± 4

Abnormal skin pigmentation Lightened 1 ± 2
External Appearance Excess mucus secretion 5 ± 6

Faecal casts (anal) 1 ± 1
Fin rot 3 ± 4



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Some tested fish with difficulty in ventilation
by gulping behaviour and selection of space
aggregated in one space near the aquarium
wall, and some fish had fin rot compared to
the non-tested fish without microplastics.
However, results did not involve any mortality
cases of fish because they were not observed
in experiments. The No Observable Effect
Concentration (NOEL) was assumed to
correspond to the highest concentration tested
which was 4.40 x 104 particles/kg similar to
the report by Hasselerharm (2022).

Evaluation of microplastic toxicity on Nile
tilapia

The percentage of fish which had abnormal
external appearance was high, followed by
abnormal swimming behaviour, abnormal
ventilatory function and lastly loss of
equilibrium (Figure 2).

Key: Btc is for butch number
Figure 2. Percentage of Fish with their response to Microplastics

The loss of equilibrium was determined by
observing fish which had shown clinical signs,
namely abnormal horizontal orientation,
abnormal vertical orientation and loss of
buoyance which was in 35% of fish. The
percentage of fish which had shown loss of
equilibrium gave scattered cycles which were
underlying straight line to show linear

relationship between the two variables. The
relation to engulfed microplastics had a large
negative correlation coefficient, r = -0.7, p =
0.14. Therefore, there was a large relationship
between percentage of fish which had loss of
equilibrium and the engulfed microplastics
(Figure 3).



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Figure 3. Relationship between fish with abnormal appearance and concentration of engulfed
microplastics

The abnormal swimming in fish was observed
in 48.1% of fish. The clinical signs which
were observed in fish for abnormal swimming
were: Hypo activity, abnormal bottom
distribution and dense schooling or shoaling.
The percentage of fish which had abnormal
swimming indicated relationship with
engulfed microplastics (Figure 3), where the
clustered cycles were close to the straight line.
The correlation coefficient, r = 0.9, p = 0.14

indicated presence of large relationship
between the percentage of fish with abnormal
swimming and engulfed microplastics.

Abnormal ventilation in test fish was observed
in 58.8% of fish. The clinical signs for
abnormal ventilation observation were: hyper
ventilation and hypoventilation. The
percentage of fish which had abnormal
ventilation indicated a relationship with
number of engulfed microplastics (Figure 3)



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where the clustered cycles were close to the
straight line. The correlation coefficient, r = -
0.4, p = 0.13 indicated the presence of
medium correlation between percentage of
fish with abnormal ventilation and engulfed
microplastics.

Abnormal appearance of test fish was not
observed in 38.5%. The clinical signs were;
oedema, fin rot, and excessive mucus on

epidermal part. The clustered cycles were
close to the straight line to show close
relationship between percentage of fish with
abnormal appearance and engulfed
microplastics (Figure 3). The correlation
coefficient, r = -0.17, p=0.13 indicated small
relationship between percentage fish with
abnormal appearance and engulfed
microplastics.

Figure 4. Responses of fish to microplastic exposure in four different experiments

The responses of fish on exposure to
microplastics was monitored daily and
results indicated that the percentage of
tested fish which had positive responses
started within day 3 to day 5 from the
day microplastics were introduced, then
increased to the maximum in 10 days

(Figure 4). Among the fish which were
exposed to microplastics, 72.5% of
them had small weight and 75% had
smaller length growth compared to fish
which were not exposed to
microplastics. The observation was
made for daily increase of wet weight
in both tested fish and non-tested fish.



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The daily wet weight increase in tested
fish was low compared to the non-

tested fish (Figure 5).

Figure 5. Change in Wet Weight of Oreochromis niloticus with days for tested fish and non-tested fish
in two different experiments.

DISCUSSION

The temperature conditions which were used
in keeping fish for study were in the normal
range which is required for fish farming and
did not differ much from the temperature
17.7ºC to 27.7ºC used by Chaudhary and
Sharma (2018) in the experiment for tilapia in
fish tank. Water electric conductivity, total
dissolved solids, and pH were within the limit
of water quality requirements where pH is 6.0-
9.0 for freshwater fish, TDS upper limit is
1500 ppm, although the one that causes effect
is 5000−10,000 ppm (Scannel and Jacobs,
2001). One report by Chaudhary and Sharma
(2018) for fresh water fish kept in the tank,
indicated that water was kept alkaline with pH
range of 8.3−8.50 in both control and test
tanks and the water electric conductivity was
1730µs/cm–1980 µs/cm, the conditions which
were different from what used in this study

but both were within the limits of freshwater
fish growth. In this study, it was found that
microplastics concentrated in gills and
intestines the scenario which indicates that the
test fish in aquarium could not manage
eliminating particles of microplastics during
taking in water through gills, and eating food.
Events like these have been reported also in
other studies for microplastics in marine biota
because of presence in ecosystems (Thompson
et al., 2009).

The responses of Nile tilapia on exposure to
microplastics in this study have been reported
as fish stress caused by toxic contaminants of
water in many literatures (Coyle, 2004;
Davidson et al., 2011; Reverter, 2018;
Sridhar, 2021). Cocci et al. (2022) reported
that microplastic occurrence in fish was found
to be correlated with antioxidant enzymes
(catalase and superoxide dismutase) and
cytokinases (interleukin 1 ß, 10 and



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65

interferon) levels, causing reactive oxygen
species (ROS) generation and immune cell
infiltration in the gut. Moreover the study by
Alimba and Faggio (2019) indicated that
microplastic presence in marine vertebrates
produces oxidative stress in proteins, lipids
and deoxyribonucleic Acid (DNA) by altering
the antioxidant defuse mechanisms, i.e.
enzyme catalase (CAT), superoxide dismutase
(SOD), glutathione-s-transferase (GST),
glutathione peroxidase (GPx) and reduced
glutathione (GSH) genes at the catalytic and
transcriptional levels. They regulate the gene
expression that controls oxidative stress by
acting as pro-oxidant stimuli activating
antioxidant gene expression through nuclear
factor erythroid 2–related factor 2(Nrf2)-
dependent mechanism. The clinical signs like
abnormal swimming which was observed in
Nile tilapia in this study has been reported
also by Wright et al. (2020) who had observed
that the polyvinyl chloride which were
exposed to brown trout had reduced
swimming activity and there was change in
their circadian rhythms. Furthermore, the
report by Tang et al. (2018) indicated that
microplastic exposure in sea bream heads
altered the JNK (c-Jun N-terminal kinase) and
ERK (extracellular signal-regulated kinase)
signalling pathways which are involved in the
detoxification process of fish. Even other
abnormal behaviours observed in Nile tilapia
in this study might be attributed to the similar
situation as Yao et al. (2023) reported for the
exposed high concentration of polystyrene
microplastics in golden pompano (Trachinotus
ovatus) which had caused oxidative damage
and up-regulation of genes (GrPn78, X6p-1,
and Elf-2α), resulting in endoplasmic
reticulum stress (ERS) and severe oxidative

stress which raised the BAX/BcL-2 ratios and
induced death. More studies report similar
cases of microplastic exposure effects
although they do not mension directly the
clinical signs which have been considered in
our observation in Nile tilapia but the
oxidative stress must be the reason. For
instance; the study by Cao et al. (2023)
reported the effects of polyethylene
microplastics in carp and found that they
elevated the expression of P53, NF-KB, P6S,
1KKα, 1KKß, caspase-3 genes in the gills.
Also the extended exposure to polystyrene
microplastics in coach juveniles
(Paramisgurnus dabbyanus) was studied by
Wang et al. (2022) and found to inhibit the
expression of keep 1-Nrf2 signalling pathway
genes inducing aptosis by upgrading proteins
(P53, gadd 45 ba and caspase 3b) expressions,
also were found to up-regulate TNF-α and
PTGS2A which are gene markers in the
inflammatory mechanism in zebra fish
(Umamaheswari et al., 2021). Lastly, the
study by Chen et al. (2021) on tilapia had
found that polyethylene microplastics caused
oxidative stress in the liver by damaging cell
membrane increasing lipid peroxidation
(PLO) levels, in the brain, dorsal muscles and
gills, higher brain activity where 32% of the
fish had microplastics in the dorsal muscle
(Barboza et al., 2020). In addition poor quality
of water that was contributed by presence of
microplastic contamination might have lead
also to diseases that rise from bacteria and
fungi. Because fouling organisms like fungi
and bacteria can easily attach to microplastics
causing them to be agents of diseases (Dekiff
et al., 2014). Furthermore, responses indicated
by clinical signs in this study have also been
observed in other studies for effects of toxin



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66

contamination where 30% to 35% of
individual fish responded positively for
example some had frequent surface movement
with gulping of air (Islam et al., 2021), the
abnormal alterations were associated with
physiological responses which were inductive
of stress.

CONCLUSION AND RECOMMENDATION

The test fish engulfed microplastics which
were found in intestines and gills. The
engulfed microplastics resulted to stress in
Nile tilapia which was observed from different
responses according to clinical signs and the
lag in proper growth. There were no mortality
of Nile tilapia but the responses to presence of
microplastics in aquarium indicated that
microplastics had adverse effects to Nile
tilapia. The evaluation of toxicity of
microplastics to Nile tilapia indicated that fish
in contaminated ponds have high
susceptibility to the effect of microplastics
which might lead to death if the conditions
have not been controlled. Although there have
been reports on the effects of microplastics as
a cause of oxidative stress to fish, more
studies need to be done to give clear
description for some observations like fin rot
and abnormal mucus secretion responses
which might be associated to microplastic
harbouring microorganism in large amount.

Acknowledgement:

Authors would like to thank the laboratory
technicians of Open University of Tanzania
and University of Dar es Salaam for their
great support in laboratory activities and
instrumental analysis.

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